The present invention relates generally to integrated circuits and, in particular, to the packaging of integrated circuits.
Electronic devices such as cellular telephones and notebook computers typically contain a number of integrated circuit (IC) packages mounted to a printed circuit board (PCB). IC packages typically include a single IC die (or chip) on a substrate or leadframe. The die and substrate are encapsulated in a material such as plastic. The encapsulated packages are then mounted to another substrate such as a PCB.
Multichip modules (MCM) are IC packages that can contain two or more integrated circuits. The size of the electronic device that uses MCMs can be reduced because MCMs typically have a number of individual IC dice mounted within a single package in a laterally adjacent manner. The outer dimensions of all the individual elements limit the minimum footprint of a multichip module, however. Moreover, multichip module substrates are typically constructed from ceramic, silicon, metal or printed circuit board materials that are relatively expensive to produce. Considerable effort has been expended to provide an electronic package that has a minimal footprint and volume and that can be assembled with conventional plastic injection molding techniques without adding expensive interconnecting substrate components.
FIG. 1 shows another type of IC package configuration that attempts to decrease the footprint and volume of the IC package. This type of IC package is known as a Stacked Chip Scale Package (SCSP). IC package 100 includes stacked IC dice. The SCSPs 101 are formed by stacking several sets of IC dice on a long substrate, wire bonding, encapsulating the IC dice, and then slicing the substrate and encapsulant to separate each SCSP 101.
Substrate 110 includes bond fingers 112 connected to conductive traces 114 on the top surface of the substrate 110. Bond fingers 112 are conductive areas on the substrate 110 that provide locations for wire bonding of the IC dice to the substrate 110. Vias 116 are conductive interconnects that extend through the substrate 110 to electrically connect traces 114 to conductive pads 117 on the bottom surface of the substrate 110. One example of a substrate is a printed circuit board (PCP). Other examples of materials for substrate 110 are: FR4, BT, tape automated bonding (TAB) tape material, ceramic, silicon on sapphire (SOS), or a multi-layered substrate such as OLGA.
The SCSP 101 shown in FIG. 1 is connected to a circuit board (not shown) by solder balls 118, which are placed on pads 117 on the bottom surface of the substrate 110. Other types of IC packages may include leads that extend laterally with respect to the dice within the package for connection to an external circuit board.
FIG. 1 also shows a first die 120 mounted to substrate 110. Second die 130 is mounted on the top surface of first die 120. An adhesive 103 such as epoxy is used to mount the die. After first die 120 and second die 130 are mounted, they are wire bonded to the substrate 110. First die 120 has bond pads 122 on its top surface near its edges, and second die 130 has bond pads 132 on its top surface near its edges. Bond wires 124 connect bond pads 122 of first die 120 to the substrate 110, while bond wires 134 connect bond pads 132 of second die 130 to the substrate 110.
FIG. 2 shows a top view of IC package 100 after wire bonding and before encapsulation. First die 120 is mounted to substrate 110. Bond pads 122 are connected by bond wires 124 to bond fingers 112. Second die 130 is mounted on top of first die 130. Bond pads 132 on second die 130 are connected to bond fingers 112 by bond wires 134. The area in the center of first die 120 limits the size of second die 130 because second die 130 cannot cover bond pads 122 of first die 120. This is especially problematic in dice having bond pads adjacent all four edges.
FIG. 3A shows an IC die configuration 200 that includes a first die 220 and a second die 230 stacked on top of first die 220. First and second dice 220 and 230 are stacked on substrate 210. Substrate 210 has bond fingers 212 adjacent two opposite ends, as shown in FIG. 2. Bond fingers 212 correspond to bond pads on first and second dice.
First die 220 has bond pads 222 adjacent two opposite edges rather than adjacent all four edges as in the first die 120 of FIG. 2. In order for second die 230 to be stacked on top of first die 220, second die 230 must fit between bond pads 222 of first die 220. Thus, the size of second die 230 is limited by the bond pad configuration on the first die 220.
FIG. 3B shows an IC die configuration 200xe2x80x2 that includes a first die 220xe2x80x2 and a second die 230xe2x80x2 stacked on top of first die 220xe2x80x2. First and second dice 220xe2x80x2 and 230xe2x80x2 are rectangular and have bond pads 222xe2x80x2 and 232xe2x80x2, respectively, near opposing short edges. Die configuration 200xe2x80x2 allows two dice that are the same size to be stacked and yet leave the bond pads of the lower die exposed, but these rectangular dice must be stacked with their respective axes perpendicular. Also, the second die 230xe2x80x2 must be narrow enough to fit between the bond pads 222xe2x80x2 of the first die 220xe2x80x2. The size of the second die 230xe2x80x2 is therefore limited.
In one embodiment, an apparatus includes a lower die having a top surface and two adjacent keep out areas on the top surface. The keep out areas are next to two adjacent edges of the lower die. The lower die further includes at least one non-bonding edge area. An upper die is stacked on the lower die such that the two adjacent keep out areas are exposed to accept wire bonds.